1. Field of the Invention
The present invention relates to recording-head substrates, recording heads using the recording-head substrates, and recording apparatuses including the recording heads. In particular, the present invention relates to a recording-head substrate used for discharging ink by, for example, inkjet technologies so as to record information, a recording head using the recording-head substrate, and a recording apparatus including the recording head.
2. Description of the Related Art
As apparatuses for outputting information used in, for example, word processors, personal computers, and facsimiles, printers for recording information, such as desired text and images, on sheet recording media, such as paper or film, are used.
Various recording technologies in printers are available. Inkjet technologies have increasingly received attention in recent years because the inkjet technologies are capable of recording information on a recording medium, such as a sheet of paper, in a non-impact manner, of operating quietly, and of easily realizing color inkjet printers. In these inkjet technologies, serial recording methods, which record information by using a recording head for discharging ink in accordance with desired recording information while moving reciprocally in a direction perpendicular to the direction in which a recording medium is conveyed, has been widely used in general because the serial recording methods can realize inexpensive and small printers relatively easily.
Among the serial recording methods, a thermal inkjet method discharges a droplet of ink by using a bubble created by thermal energy generated by passing a current through a heater in contact with ink for several microseconds. According to this method, many nozzles can be arranged at high density in a recording head. This is advantageous in view of improvements in recording speeds, and therefore, this method has received much attention.
A recording head in a recording apparatus according to such a thermal inkjet method uses an element substrate in which a heater for heating ink, a protective film for the heater, a driving circuit for passing a current through the heater, a logic circuit for controlling the driving circuit, and the like are formed integrally on a single-crystal silicon semiconductor substrate by the same process for producing a semiconductor integrated circuit. Hereinafter, this recording head and this element substrate are referred to as “a recording head” and “a heater board”, respectively.
An example of the control of such a heater board in a recording head in a related art is described below.
FIG. 8 is a block diagram schematically showing the structure of the heater board in the related art.
Referring to FIG. 8, a heater board 300 includes a heater array (represented as “128 bit Heater” in FIG. 8) 301 having 128 heaters in sequence. The number of the heaters is not limited to 128. A plurality of heater arrays, each having the same number of heaters, facing each other may be used.
In FIG. 8, the heater array 301 is connected to a driver array (represented as “128 bit Driver” in FIG. 8) 302 having as many drivers as the heaters. The drivers are individually connected to the heaters so as to drive each of the heaters. The heater is a thin-film resistor having a resistance of several tens to several hundreds of ohms. The driver array 302 includes high-voltage power transistors having withstand voltages required for passing a current of several tens to several hundreds of milliamperes, and the number of these power transistors is the same as the number of heaters (128 in this example). Commonly, such a power transistor requires a withstand voltage for a dozen or so volts to several tens of volts.
The driver array 302 is connected to AND gates 303 for determining the on or off state of each of the drivers. A driver connected to an AND gate that produces a true output is selected in order to switch a corresponding heater to the on position.
An input terminal 306 is used for applying a signal (heat-enabling signal) for specifying a time for passing a current through the heater. For the time for which the heat-enabling signal is applied, a driver selected by the AND gates 303 is activated and a current flows through a corresponding heater.
In this example, as shown in FIG. 8, the AND gates 303 for selecting the drivers each have two input terminals. One input terminal of each of the AND gates 303 is connected to the output from a decoder (represented as “DECODER” in FIG. 8) 304. The decoder 304 deals with a 4-bit input and a 16-bit output. One input terminal of each of the AND gates 303 is connected to any one bit of the 16-bit output. The other input terminal of each of the AND gates 303 is connected to any one bit of the output (8-bit output) of a 12-bit shift register 305 (represented as “12 bit S/R”).
The remaining four bits of the output from the 12-bit shift register 305 are connected to an input terminal of the decoder 304, so that these four bits of data are decoded into a 16-bit signal in the decoder 304.
The 16-bit output from the decoder 304 and the 8-bit output from the 12-bit shift register 305 are subjected to AND operation so that a desired heater among the 128 heaters is selected and driven.
The 128 heaters are driven in a time shared manner in units of 16 sections divided by using the 16-bit output from the decoder 304. The maximum number of heaters simultaneously driven is eight. This occurs when all eight bits of output data from the 12-bit shift register 305 are determined to be true.
Signals applied when data is transferred to the 12-bit shift register 305 will now be described.
The 12-bit shift register 305 is connected to a data-signal input terminal 307, a clock-signal input terminal 308 for a clock signal to indicate a timing for capturing data, and a latch-signal input terminal 309 for a latch signal to indicate a timing for temporarily storing transferred data so as to receive three types of signals from these input terminals.
Serial data (DATA) to indicate a nozzle to be driven is input from a main system of a recording apparatus including the recording head via the data-signal input terminal 307 in synchronization with a clock signal (CLK) applied to the clock-signal input terminal 308. Inside the heater board, the serial data (DATA) is transferred to the 12-bit shift register 305 in synchronization with the clock signal (CLK), and the transferred serial data (DATA) is converted to parallel data at the 12-bit shift register 305. The parallel data is temporarily stored in a latch circuit (not shown) for storing data in accordance with the latch signal (LATCH) applied to the latch-signal input terminal 309.
The data corresponding to the last four bits of the serial data in the stored data is decoded at the decoder 304. The decoded data and the data corresponding to the first eight bits of the serial data are subjected to an AND operation at the AND gates 303, so that a desired driver of the drivers corresponding to the 128 heaters is selected.
In this example, the heaters are driven in a time shared manner, as described above, and therefore, the number of heaters selected simultaneously in the 128 heaters is limited to eight.
When a logical operation is determined so as to be ready to activate only a driver corresponding to a desired heater, as described above, a logical signal (heat-enabling signal (HENB)) to specify a time for passing a current through a heater from the main system of the recording apparatus is applied to the input terminal 306, so that a current is passed through only a heater corresponding to a desired nozzle for a time for which the heat-enabling signal is applied.
Input buffers 310 for shaping a waveform of a signal for driving an internal circuit are arranged adjacent to each pad of the input terminals (see, for example, Japanese Patent Laid-Open No. 8-108550).
In general serial printers, the length of the heater array is equal to the length of a recordable area in one pass of the recording head in the paper feed direction. In other words, recording is performed while the recording head is moved across the width of the recording medium; the recording medium is then advanced by the length of the heater array of the recording head in the direction to be recorded; recording is again performed while the recording head is moved. This process is repeated until the entire recording medium is recorded. Depending on the circumstances, moving the recording head is performed multiple times over a predetermined area of the recording medium (multiple-pass recording) so as to improve the recording quality.
In this type of printer, one requirement is to increase the recording speed. To this end, the number of heaters arranged is increased and the length of the heater array is extended, so that a recordable area of the recording medium in one pass of the recording head is increased. In addition, the length of the heater array is made equal to the width of the recording medium so that recording on the entire recording medium is performed at one time without moving the recording head. This type of the recording head (full-line-type recording head) further improves the recording speed.
In addition to increasing the number of heaters and the length of the heater array, for an inkjet recording method, the period of discharging droplets of ink from a nozzle corresponding to a heater of the recording head is reduced (the frequency of discharging droplets of ink is increased), thus improving the recording speed. In order to increase the length of the heater array (i.e., nozzle array) for further improvement in the recording speed, it is necessary to increase the physical length of the heater board.
In order to increase the number of nozzles, an increase in the number of gates of logic circuits inside the heater board to control the nozzles is required. In order to increase the frequency of discharging ink, the operating speed of the logic circuits must be increased correspondingly.
Generally, in a heater board, a logic circuit for controlling the heater board is included in an integrated circuit using a semiconductor process. Therefore, an increase in the length of the heater board itself results in an increase in the line length of the logic circuit inside the heater board. This leads to a characteristic problem with the heater board used in the recording head. Specifically, even if an integrated circuit technology that achieves a high operating speed by a fine-line circuit process is used, a recording head that has an increased line length in a circuit to improve the recording speed has a problem in that the operating speed of the circuit is decreased because the adverse effects of parasitic resistance and parasitic capacitance in the lines become unignorable.